In semiconductor chip fabrication and packaging, the steps of etching different layers are among the more critical and crucial steps. Methods that are commonly used for the selective etching of organic polymers, including photoresists and dielectric substrates, include dry etching processes such as oxygen plasma etching and reactive ion etching in an oxygen containing plasma.
Oxygen plasma etching is a procedure that generally involves filling a container such as a bell jar with gas capable of providing chemically reactive oxygen ions. The substrate which is to be etched is covered by a mask and introduced into the container along with the reactive gas. The reactive gas is usually disassociated forming positive and negative ions by coupling radio frequency power to the plasma by a capacitive or inductive coupling. It is believed that the disassociated ions then chemically interreact with the surface to be etched. In such a process, the substrates are positioned either on a ground plane or at the same potential as the plasma gases.
In reactive ion etching, generally a container such as a bell jar is filled with a gas capable of providing chemically reactive oxygen ions. The cathode is negatively biased relative to the anode, for instance, by means of an applied radio frequency signal. The surface to be etched is covered by a suitable mask and is then placed on the cathode. By applying an electric field, the reactive gas disassociates and the chemically reactive oxygen gas ions are attracted to the cathode. It is believed that the surface is etched both by chemical interreaction with the active ions and by the momentum transfer of the ions impinging on the surface.
These etching processes employing an oxygen containing plasma have particular application for pattern transfer into organic polymeric dielectric substrates. In similar manner, oxygen containing plasma processes are used to clean polymeric surfaces to remove impurities therefrom. The organic polymeric substrates readily oxidize under oxygen containing plasma conditions forming volatile products that are easily removed. However, surface oxidation is known to occur (in varying degrees), wherever the polymer is exposed to the plasma process including on the walls of etched vias. Likewise, dry etch chemistries leave behind a surface which is qualitatively different from that obtained by wet etching. Accordingly, subsequent process steps that depend upon the nature of such surface are effected.
For successful substitution of dry etch technology into process flows which previously utilized wet etch technologies, it is necessary that the polymeric surface be compatible with the next, previously-determined process step or that new subsequent process steps be developed. Since oxygen plasma and oxygen reactive ion etching processing of various organic polymeric materials may result in the ubiquitous surface oxidation of the exposed polymeric material, many processes employed following wet etching techniques fail when attempted using dry etch processes. By way of example, subsequent plating of a polymeric substrate including polyimides employing an electroless plating bath is problematic.